An NSOM is a type of microscope that is used to image objects of different sizes in a sub-wavelength region of light with resolution greater than those achievable by conventional optical microscope. It is well known to those skilled in the art that the minimum resolution of conventional optical microscope is limited by the diffraction limit, expressed as d=0.61λ/NA, where λ is the vacuum wavelength; NA is the numerical aperture of the optical component (usually 1.3-1.4 for modern objectives). Therefore, the resolution limit for conventional optical microscopy is usually around λ/2. The diffraction limit assumes that the light is diffracted into the far-field that propagates without any restrictions, in comparison, an NSOM makes use of evanescent or non propagating fields that exist only near the surface of the object. As a result, the NSOM can obtain resolutions that are better than those obtained by using conventional optical microscopes. To work in the near field region, NSOM requires a small source of light placed close to the sample to be scanned. A source of light having a diameter much smaller than the wavelength of light is used. Further, the distance between the sample and the microscope tip is kept much less than the wavelength of light.
Applications of NSOMs include, but are not limited to, single-molecule spectroscopy, Raman spectroscopy, data storage, high-speed imaging, and experiments using measuring instruments in the fields of nanotechnology, biotechnology, optical technology, meso-scopic physics, chemistry, etc.
It is known to those skilled in the art that the typical NSOM tip is formed by coating the tip of an optical fiber with metal. Typically, the optical tip of an NSOM has two components: a core and a cladding. The core is usually made of an optical fiber. The cladding, which surrounds the core, is usually a metal such as aluminum. The sample is illuminated by directing a beam of light through the core of the optical tip. For example, a laser source is used to illuminate the sample. The light passing through the sample is collected and detected. Alternatively, the light reflected from the sample is collected and detected. The sample can also be scanned by illuminating it with an external source of light, then collecting and detecting the reflected or transmitted light through the optical tip.
When the sample is illuminated by directing a beam of light through the core of the optical tip, the amount of energy transmitted from the light source to the optical tip is referred to as the throughput of the NSOM. The throughput of NSOM can also be described as the percentage of light reflected from the sample surface and collected by the optical tip, which can be transmitted to the other end of the optical fiber for detection purpose. In current NSOMs, there is a large loss of light energy inside the optical tip due to the design of the optical tip. Most of the optical energy provided through laser source is absorbed inside the optical tip. As a result, the throughput of the NSOM is very low. For example, in an NSOM application with an optical tip aperture of 50-200 nm, the wavelength of light used is 400-500 nm, light throughput is around 10−1 to 10−4 percent.
FIG. 1A illustrates a Near-field Scanning Optical Microscope (NSOM) 100 set-up in the transmission mode of imaging, in accordance with various embodiments of the invention. NSOM 100 is a microscope that is used for imaging objects of various sizes in the sub-wavelength region of light. NSOM 100 comprises an optical tip 102, an optical fiber 104, a light source 106 and a light detector 108. Light-source 106 generates a beam of light. Light-source 106 is a ‘Light Amplification by Stimulated Emission of Radiation’ (LASER) source. The beam of light is passed through optical fiber 104 to optical tip 102.
A sample 110 that is to be scanned is placed under optical tip 102. Sample 110 is illuminated by the beam of light directed by optical tip 102. This beam of light is transmitted through sample 110 and is collected and detected at light detector 108. The collected light contains optical information pertaining to sample 110. Thereafter, the optical information is analyzed.
The collection and detection of the light passing through sample 110 by transmitting the light through sample 110 is referred to as ‘transmission mode imaging’. The optical information can also be collected by other modes of imaging such as the reflection mode, the collection mode or the illumination-collection mode.
NSOM 100 can work in the reflection mode of imaging as shown in FIG. 1B. In the reflection mode of imaging, sample 110 is illuminated by using a beam of light coming through optical tip 102. The light reflected from sample 110 is collected and detected at light detector 108 that is placed about sample 110. In this mode, optical tip 102 is only used to illuminate sample 110, and the light reflected from sample 110 is collected at light detector 108.
NSOM 100 can also work in the collection mode of imaging as shown in FIG. 1C, whereby sample 110 is illuminated by using an external macroscopic light source from above or beneath sample 110. The light reflected from sample 110 is collected at light detector 108 through optical tip 102. In this mode, the external light source is used to illuminate sample 110 and the light reflected from sample 110 is collected by optical tip 102. Thereafter, the optical information is transmitted to light detector 108 through optical tip 102.
NSOM 100 can also work in the illumination-collection imaging mode as shown in FIG. 1D, whereby sample 110 is illuminated by using a beam of light coming through optical tip 102 wherein 105 is an optical beam splitter with light transmissivity and light reflectivity so that light source 106 can be transmitted through beam splitter 105 into the optical fiber 104 and exit tip 102. The light reflected from sample 110 is collected through optical tip 102 and reflected to light detector 108 by beam splitter 105. In this mode, optical tip 102 is used to illuminate sample 110 and also used to transmit the collected light to light detector 108.
Optical tip 102 typically comprises a core and a cladding. The core is surrounded by the cladding to provide optical properties. The core is made of optical fiber. Examples of optical fiber include a glass and a plastic fiber. The material of the cladding includes metal coatings such as aluminum, silver, nickel, Gold, etc. The aperture diameter of optical tip 102 is less than the wavelength of light originating from light source 106. For example, the aperture diameter of optical tip 102 is in the range of 50 nm to 200 nm for a light source of wavelength in the range of 400 nm to 500 nm. Further, the distance between optical tip 102 and sample 110 is maintained as less than the wavelength of light. The aperture diameter of optical tip 102 governs the resolution of the image of the sample, obtained by NSOM 100.
The amount of energy transmitted from light source 106 to optical tip 102 is referred to as the throughput of NSOM 100, which depends on the optical and mechanical properties of the fiber used as the core and the metal used as the cladding. Typically, when a laser with a wavelength in the range of 400 nm to 500 nm is used as light source 106, optical tip 102 with an aperture diameter in the range of 50 nm to 200 nm and length of 10 μm is used to reduce the mode size from 8 micron to 200 nm. Optical tip 102 has a throughput in the range of 10−1 to 10−4 percent. For example, for a 532 nm wavelength of light, optical tip 102 typically provides a power output of 10 nW-100 nW for an input of 50 mW.
A high throughput at the optical tip results in faster scanning speeds. By using high-energy lasers in NSOM applications, higher energy can be obtained at the optical tip. However, this causes localized heating at the optical tip, resulting in the cladding getting damaged. Moreover, heating inside the optical tip may result in the degraded performance of NSOM over a period of time. Therefore, high-power lasers are not preferred for achieving higher power at the optical tip.
Metal-coated near-field NSOM probes can be manufactured by using the wafer-processing technique. The throughput can be increased by about one order of magnitude by preferring this technique over traditional NSOM. However, the increase in throughput does not produce the high power required at the optical tip. Additionally, each NSOM probe manufactured needs to be individually inspected for pinholes and other manufacturing defects. As a result, the manufacturing cost of NSOM probes is high. Consequently, large-scale production of NSOM probes is not possible.
In light of the foregoing, there exists a need for a near-field optical tip that minimizes energy loss inside the probe. The optical tip should have a high energy throughput for near-field scanning operations, while localized heating at the probe should be low. Further, the optical tip should enable the NSOM to achieve faster scanning speeds. Additionally, the optical tip should be easily mass-producible, should require minimum inspection during manufacturing, and should be low in cost.